This invention relates to a transmission channel for the electrically isolated transmission of digital signals.
Prior-art transmission channels of this kind are frequently used for the transmission of digital signals by means of signal transmission lines, particularly over great distances, and serve to separate the potentials between the transmitter and receiver units of a data communications system. Such potential isolation is necessary to suppress interference signals caused by transient currents on the signal transmission lines.
Because of their high efficiency, transformers are often used to provide electrical isolation. Due to the effect of the inductances of transformers, however, the use of the latter reduces the edge steepness of the digital signal, so that this method may be unsuitable. In addition, transformers also transfer electromagnetic interference introduced to them, and the suppression of such interference involves a considerable amount of technical complexity. DE-A 36 14 832, WO-A 89/12 366, and EP-A 198 263 each disclose a transmission channel for transmitting digital signals—with a first digital-signal port for a first digital signal to be transmitted and—with a second digital-signal port for a transmitted second digital signal. The transmission channel comprising:—an isolating path with a predeterminable isolation capability;—a first conversion stage; and a second conversion stare. The first conversion stage including a coupling-signal port for a first coupling signal transmissible across the isolating path. The second conversion stage including a coupling-signal port for a third coupling signal transmissible across the isolating path. The isolating path being provided between a coil of the first conversion stage and a coil of the second conversion stage. The second conversion stage converting the second coupling signal to the second digital signal by means of a flip-flop having a set input coupled to the second coupling-signal port and having an output coupled to the second digital-signal port.
If signals are to be transmitted between two transmitter/receiver units, such electrically isolating circuit arrangements must also be operable bidirectionally, i.e., in a first and a second direction of transmission. This is not possible with the prior-art transmission channels described.
Another disadvantage associated with the use of flip-flops in such transmission channels is that their on state is not defined.